PROJECT SUMMARY P3, O'CONNOR Hypertension affects ~33% of adults in the U.S. and regardless of sex, fewer than 40% of hypertensive patients taking medication achieve blood pressure (BP) control to recommended levels. Damage-associated molecular patterns (DAMPs) are released by injured cells and immune cells. High mobility group box 1 protein (HMGB1) is a DAMP, and circulating HMGB1 levels are increased in patients with hypertension. A critical barrier to improving BP control rates is lack of understanding how activation of the innate immune system alters renal function. The goal of P3 is to determine whether DAMPs act on the renal medullary circulation to cause a pro-hypertensive shift in pressure-natriuresis. Studies are based on the novel concept that spontaneously occurring rhythmic contractions of descending vasa recta (DVR) pericytes help prevent red blood cell (RBC) blockage of these long, low pressure capillaries. Our central hypothesis is that is that HMGB1 stimulates inappropriate nitric oxide (NO) production by DVR endothelial cells in low sheer states, which is detrimental as it inhibits spontaneous rhythmic contractions of DVR pericytes that normally act to prevent RBC aggregations. Further, elevated levels of circulating HMGB1 in SHR may stimulate chronic inflammation and activation of the DVR endothelial cells by acting on TLR4 receptors. This would also promote vascular clogging by slowing the movement of immune cells or by increasing vessel hematocrit secondary to plasma leakage. RBC occlusion of DVR then leads to rarefaction of surrounding medullary vasculature, impaired pressure-natriuresis and hypertension. We will test our hypothesis via three specific aims: 1) Test the hypothesis that low flow stimulates rhythmic pericyte contractility and that this is inhibited by HMGB1-induced NO production, 2) Test the hypothesis that elevated levels of circulating DAMPS in SHR promotes chronic activation of vasa-recta endothelial cells and vascular inflammation, and 3) Test the hypothesis that DAMPs promote RBC aggregation in DVR and that this contributes to rarefaction of the medullary capillaries, a pro- hypertensive shift in intrinsic renal pressure natriuresis and hypertension. Project 3 is highly synergistic with the other Projects and is dependent on all Cores for the successful completion of our aims. P1 will utilize renal vessels from experiments in P3. P2 and P3 will collaborate to investigate the role of adhesion molecules and circulating immune cells on the medullary circulation as well as whether apoptosis of medullary microvessels contributes to greater Tregs in females. The proposed studies utilize a highly integrative approach incorporating in vivo whole animal studies and ex vivo measurements of the medullary circulation to rigorously test our hypothesis. Our results will provide a mechanistic link between the seemingly disparate processes thought to be critical in the development of hypertension; activation of the innate immune system and renal medullary ischemia and have the potential to impact the treatment, not only of hypertension, but a number of diseases in which DVR blockage occurs.